ABSTRACT: Sedimentologic and Stable Isotopic Evidence Regarding the Origin of Hydrocarbon Source Rocks in the Guttenberg Limestone (Middle Ordovician), Eastern Iowa

LUDVIGSON, GREG A., BRIAN J. WITZKE, LUIS A. GONZALEZ, and ORRIN W. PLOCHER, Iowa DNR-Geological Survey Bureau and Department of Geology, University of Iowa, Iowa City, IA, and STEPHEN R. JACOBSON, Chevron Oil Field Research Company, La Habra, CA

The Guttenberg Limestone (20 ft), the middle member of the Decorah Formation, is characterized by thin intercalations between carbonates and brown shales (to 43% TOC; containing the organic-walled microfossil Gloeocapsamorpha prisca). Nodular carbonates contain G. prisca with three-dimensional preservation, but lack compacted fabrics of anastomosing shale interbeds, suggesting early diagenetic control on bedding styles. Open-marine benthic faunas (brachiopods, trilobites, echinoderms, and bryozoans) occur through the Decorah. These strata accumulated in environments below fair-weather wavebase, and the Guttenberg records decreasing tempestite frequencies relative to underlying/overlying units. Articulated echinoderm faunas in this interval suggest rapid burial by suspended muds in dep sitional realms below maximum storm wavebase. The Guttenberg is the maximum transgressive unit in the Decorah transgressive-regressive cycle.

A previously documented positive isotope {13}C excursion in kerogen and carbonate fractions of the Guttenberg was evaluated through chemostratigraphic studies of carbonate components in the sequence. Results show the carbonate isotope{13}C excursion resides in micrites, while coexisting nonluminescent brachiopods record invariant marine carbonate isotope{18}O and isotope{13}C compositions. Temporally invariant compositions in the shelly benthos show the isotope{13}C excursion originated in a stratified seaway. The excursion resulted from increased productivity stimulated by quasi-estuarine circulation, with primary production occurring in a hyposaline surface layer fed by runoff from the Transcontinental arch. "Photic pumping" of {12}C to the sea floor, by settling of phytoplankton, l d to {13}C enrichment in the surface layer. Isotopic data may suggest pelagic origin for micrite, although some of the micrite excursion is a diagenetic feedback of changes in isotope {13}C of organic matter in the sediment.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)